ELECTRON-TRANSFER THROUGH THIN ANODIC OXIDE-FILMS DURING THE OXYGEN EVOLUTION REACTIONS AT PT ELECTRODES .1. ACID-SOLUTIONS

The electron transfer process, which occurs during oxygen evolution at Pt electrodes from water molecules in the double layer, is analyzed. The process proceeds by electron tunneling through an insulating anodic Pt oxide film to the underlying Pt metal as oxygen is evolved. The rate equation for the reaction is based on a model of quantum tunneling through a barrier which is a composite of both the oxide film and the inner Helmholtz layer. For a barrier thickness of 10 angstrom, the probability of tunneling decreases by a factor of three as the electrode potential increases from 1.29 to 2.01 V vs. RHE. This decrease is small when compared to the observed, ca. 10(6), increase in current density for the same potential span. The current density is controlled primarily by the distribution of electron energy states of the reacting donor species which are water molecules in the inner Helmholtz plane. At a constant potential, the tunneling probability depends exponentially on the thickness of the potential barrier, and therefore the rate of the oxygen evolution reaction is strongly dependent on the Pt oxide film thickness.